1. Field of the Invention
The present invention relates to a method of detecting radioactive substance, and more particularly, to a method of detecting radioactive substance utilizing a stimulable phosphor.
2. Description of the Prior Art
Heretofore, as a method of measuring a radiation emitted by a liquid sample for detecting a radioactive substance (substance containing a radioisotope) contained therein, there are well known a liquid scintillation counting method wherein the radiation from the sample is converted into fluorescence to be detected by adding to the sample a liquid scintillator composed of a solute (fluorescent agent) dissolved in an organic solvent. In this method, the radioactivity of the radioactive substance contained in the sample is measured by causing the scintillator to absorb at least a portion of radiation energy emitted by the radioactive substance and detecting light (fluorescence) emitted by the scintillator.
The liquid scintillation counting method can be applied to the case where a liquid sample containing a radioactive substance can be continuously or intermittently supplied. The measurement of a radiation emitted by the sample is carried out by collecting a given amount of the sample and adding the liquid scintillator thereinto.
As a method for separation analysis, for example, there is known liquid chromatography wherein a sample solution is introduced into a column packed with a filler such as an adsorbent, an appropriate solvent is then supplied therethrough to develop the sample, and components in the sample are subsequently allowed to flow out from the column and collected. The liquid chromatography can be used for the separation of a sample containing radioactive substance, and the separation and identification of the radioactive substance are conducted by measuring a radiation emitted by an eluate collected through the operation of the liquid chromatography.
More in detail, the separation and identification of a radioactive substance in a sample are conducted by collecting the sample containing the radioactive substance which is separated and developed in the liquid chromatography with a fraction collector, adding a liquid scintillator to each fraction, and then detecting light emitted by the scintillator with a photomultiplier to convert it into an electric pulse and counting the electric pulse to measure the radiation dose of each fraction.
The above-described liquid scintillation counting method has such an advantage that the radioactivity can be detected even when a radiation from a radioactive substance are weak ones such as .alpha.-rays and .beta.-rays, and is a useful means for measuring the radioactivity of the liquid sample.
The mechanism of scintillation in the liquid scintillation counting method is described as follows: A molecule of a solvent in which a solute (fluorescent agent) is dissolved is initialy excited by a radiation emitted by a radioactive substance in a liquid sample, and then a solute molecule (scintillator) is excited through impingement of the excited solvent molecule on the solute molecule, or the like. In the course of transferring of the radiation energy from the solvent molecule to the solute molecule, there occur other phenomena such that the energy is transferred between the solvent molecules by interaction between the solvent molecule in the excited state and that in the ground state, or that the energy is transferred from the solvent molecule in the excited state to the other solute molecule than a scintillator by interaction therebetween before the scintillator is excited. The transfer of energy takes place not only through the interaction between molecules such as impingement, but also through such a phenomenon that the scintillator absorbs light emitted by the excited solvent molecule or other excited solute molecule.
However, in the course of the energy-transfer procedure, there also occurs such a quenching phenomenon that the excitation energy is absorbed by a portion of the solvent molecules or other solute molecules so as to be converted into heat, etc., or that the light emitted by the scintillator is absorbed by a light-absorbing substance contained in the sample.
The liquid scintillator, which is essential to said liquid scintillation counting method, is expensive and has to be isolated and refined to re-use it. Usually, it is difficult to recover the scintillator in a high purity so that it is not generally re-used and the measuring cost is increased thereby. Further, there are problems in handling thereof. For example, the used scintillator containing the radioisotope may cause a problem in the disposal stage such as environmental pollution.
Other problems reside in that the solvents allowed to use in conjunction the solutes (fluorescent agent) are generally limited to certain organic solvents so that there is difficulty in choosing a solvent for a sample, and specific procedure has to be taken in preparing a sample in the case that the sample is sparingly soluble in the solvent.
Since the scintillation mechanism of the liquid scintillator is complicated as stated above, the counting efficiency (that is, intensity of radioactivity to be detected) is apt to decrease by the quenching effect due to impurities contained in a sample or of the sample per se. For example, light emitted by the scintillator is liable to be quenched by the oxygen dissolved in the liquid scintillator, or to be absorbed (i.e. quenched) by a colorant in the case that the sample solution is colored thereby. In the case that a sample is sparingly soluble, it is not easy to prepare a sample solution in a homogeneous phase, and the unhomogeneous phase thereof causes internal absorption of a radiation emitted from the sample. For this reason, it is necessary to accurately determine the counting efficiency of the sample by making correction for quenching caused by the above-mentioned various phenomena, and this makes measuring procedure more complicated. Further, there is a disadvantage that it is substantially difficult to accurately measure the radioactivity of the sample even if the correction for quenching is made.
In order to prevent the counting efficiency from being lowered by quenching effect of contaminants, impurities and colored substances contained in the sample, the sample has to be carefully prepared, and high skill and much experience are required for the operators. The pretreatment of the sample to remove the contaminants is of importance to the measuring procedure.
In the conventional scintillation counting method, the measurement of radioactivity of a sample is conducted in a real time. Namely, it is necessary to continuously measure light emitted by the scintillator for a given time (for example, for several minutes to several ten minutes) after the sample is introduced into the scintillator solution. If the intensity of radiation from the sample is low, the measuring time (i.e., counting time) extends to a long time so that it can be hardly said that the measuring efficiency and the handling efficiency of measuring apparatus are sufficiently high. Therefore, in the case that a great number of samples are involved as described above, it is difficult to treat such a great number of samples once and the waiting period accordingly extends for many hours so that it disadvantageously takes a long time until the results are obtained. Particularly, in the case that the half-life of radioisotope in the sample is short, it becomes difficult to measure the radioactivity thereof. Further, in the case that the radiation intensity is low, the measurement becomes more difficult. This means that an apparatus (for example, dark current drift of photomultiplier) to be used must be stable over a long period of time. To keep the stability of apparatus, an expensive apparatus becomes necessary, or much skill and experience to adjust the apparatus are required.
Further, when the conventional liquid scintillation counting method is applied to a liquid sample containing a radioactive substance which is supplied continuously or intermittently from the above liquid chromatography, the radioactivity must be detected for every fraction by collecting the sample (which has been separated and developed in the liquid chromatography) by means of a fraction collector composed of a plurality of measuring containers (vials), and then measuring the radiation dose of each container with a scintillation counter.
Accordingly, in order to separate and identify the radioactive substance in the sample with higher accuracy by detecting the radioactivity of the separated and developed sample, it is required to use a greater number of measuring containers as the fraction collector. This means that the measuring procedures for collecting the sample solution and detecting the radioactivity are made complicated.